Fixes: IM plugin consumes touch events of other IM plugins
Fixes: IM plugin consumes touch events of other IM plugins

RevBy: Michael Hasselmann
Details: All IM plugins share the same QGraphicsScene. Meego-keyboard installs
a (fullscreen) MSceneWindow as a root item in the MPlainWIndow instance.  If
this window is still visible while switching plugins, it will interfere with
event processing plugins.  This commit appears and disappears the scene window
properly, providing a temporary workaround for the shared-QGraphicsScene issue.

KDE/plasma-maliit-plugins,RHawkeyed/plugins,RHawkeyed/plugins,KDE/plasma-maliit-plugins,RHawkeyed/plugins,KDE/plasma-maliit-plugins,RHawkeyed/plugins,develersrl/maliit-demo-qml,maliit/plugins,develersrl/maliit-demo-qml,KDE/plasma-maliit-plugins,develersrl/maliit-demo-qml,develersrl/maliit-demo-qml,RHawkeyed/plugins,maliit/plugins,maliit/plugins,maliit/plugins